In the field of vibration testing, electromagnetic actuators, also called shakers, are typically used in the production environment to test items at varying levels of force, velocity and displacement and over varying periods of time. Some shakers are constructed to apply low levels of force and over relatively short periods of time while others are made for more extreme conditions, such as are necessary for shock testing. To be suitable to test items under heavy loads and at very high stress levels over long continuous periods, shakers must be extremely robust and highly reliable.
An example of a known electromagnetic actuator is the applicant's existing V964, V984 and V994 models. FIG. 1 illustrates a general construction of a known actuator. The armature 1 is adapted to vibrate relative to the body or stator 2 and is suspended from the stator by suspension members 7. The armature 1 includes armature coil 4 covered in a carbon fibre sheet, which is located in an annular air gap. Two electromagnets (5,6) running in opposition are provided which generate D.C. magnetic fields across the air gap to supply the motive force. The coil 4 is energised by an alternating current so that it moves relative to the stator 2, causing the armature 1 to vibrate at the frequency of the applied alternating current. An article to be vibration tested may be placed directly on top of the armature normal to its axis of vibration, or on a work table carried by the armature. Alternatively, the article to be tested may be placed on a horizontal table coupled to the armature when horizontal vibration testing is to be carried out. An example of such horizontal vibration testing is described in U.S. Pat. No. 4,489,612.
While this arrangement is generally suitable for many applications, there are problems with its overall structure and operation.
In many instances of vibration testing, pure vertical acceleration is required, with no, or at least minimal, rotational force. This is particularly the case when testing apparatus that are sensitive to rotational forces, such as gyroscopes. The armature coil is generally constructed by bonding together adjacent turns of current carrying conductors. Such conductors may be hollow to allow the passage of water for cooling purposes. The helical form of the armature coil 4 induces rotational forces. This is because, with such helical coils the current through the coil is not perpendicular to the thrust axis of the actuator, as the flux is parallel to the axis of the coil and hence of the vibrator itself. This produces a torsional force at an angle to the axis of the vibrator and which excites resonance in the coil 4. This resonance also causes rotational resonance in the armature 1, which is generally undesirable.
The size and construction of the armature illustrated in FIG. 1 is also inflexible as its size is dependent upon the size of the coil arrangement. This is because in these actuators there is a single flux gap with two field coils providing the motive force. These run in opposition to concentrate the flux. In operation the relative movement between the armature 1 and the stator 2 results in the armature breaking and disturbing the magnetic fields generated between the magnets. FIG. 3A illustrates the magnetic field lines of the actuator shown in FIG. 1. As can be seen from FIG. 3A, the magnetic fields in this arrangement are quite complex and there is much competition for space in the magnetic circuit. In order to minimise the disturbance to the fields, the armature structure has to be long and webbed. As the armature structure undergoes large stresses, it is generally desirable to have flexibility in designing its construction in order to optimise its performance.
Another problem with such electromagnetic actuators is the location of the electrical and coolant connections and armature bearings of the structure. FIG. 1 shows the centrally located bearing 8, which allows movement of the armature along its vibratory axis. The electrical/cooling connector for the coil is shown in FIG. 3A at 27a, and a corresponding electrical/cooling connector 28a is located on an upper edge of the coil. These connections are therefore also located within the body of the actuator. Particularly for maintenance purposes, the positioning of these components within the actuator body, while necessary in view of the structure of the actuator, means that it is necessary to at least partially dismantle the actuator to obtain access to the components, which can be a time-consuming operation.